In digital to analog converters ("DACs"), various types of devices including field effect transistors ("FETs"), bipolar junction transistors ("BJTs") and heterojunction bipolar transistors ("HBTs") may be configured in differential pairs as current switching devices. Typically, the transistor pair operating as a single-pole-double-throw switch directs current flow from one of two, alternative paths to a precision current source depending on the logic state of an incoming binary signal (i.e., "high" or "low" state). The output of a plurality of such switches is an analog signal that represents the input binary logic signal. Typically, the number of switches used in a DAC corresponds to the number of bits to be converted to an analog signal. For example, eight bit switches (transistor-pairs plus precision current sources) may be used to convert an eight bit, binary input signal to an analog output signal.
In manufacturing integrated circuit DACs, current or voltage switches may be fabricated by forming them as HBT devices in a heterogeneous integrated-injection logic ("HI.sup.2 L") gate structure. Since the structure of the HBT devices is compatible with the HI.sup.2 L gate structure, the bipolar transistors may be readily formed on the HI.sup.2 L integrated circuit chip during the fabrication process.
In an integrated circuit DAC manufacturing process, bipolar transistors configured as current or voltage switches may be formed in a Gallium Arsenide ("GaAs") HI.sup.2 L gate structure. In such a structure, the switching transistors are formed with all of their emitters in common and connected to the conducting substrate. Consequently, this circuit requires only one power bus, which makes it one of the densest among the GaAs circuit technologies. However, since the switching transistors' emitters are formed in common on a single substrate, each individual emitter is inaccessible. Therefore, without access to each emitter, differential pairs cannot be constructed and accurate (i.e., reduced error) current mode switching is difficult to implement in a GaAs I.sup.2 L integrated circuit.